This invention relates to a digital wireless wideband RF communication system operating in periodic noise environments. Such a system is useful, for example, for communicating digital data at high data rates; e.g. rates greater than 1 Mega bits per second (Mbps).
With the advent of digital photography, the wireless transmission of digital images, for example between a portable transmitting device like a digital camera and a receiving device such as a personal computer or other image appliance such as a printer, has become a desirable feature. High data rate transmissions are desirable because digital images represent a large amount of data and short transmission times are needed. Short transmission times result in shorter wait times while an image is being transferred from a camera to a receiver and in reduced battery power consumption.
Although there are several useful communication bands available for such a digital communication system, the 2.4 to 2.5 GHz ISM Band is particularly attractive because the band is unlicensed and available internationally. However, a major problem exists with the use of this band for wireless communication. The band is allocated for microwave ovens and other devices, which as described below, generate a great deal of periodic noise in the band.
Radio frequency (RF) transmissions in the 2.4 to 2.5 GHz ISM Band have historically had to deal with the presence of manmade noise from microwave ovens that predominately operate in the center of this band at 2.45 GHz. The noise emanates from the oven by leakage through the enclosure of the oven. The leakage noise is periodic and has a radiated output power approximately 20 dB greater in strength than that allowed by the FCC for operation of Part 15 non spread spectrum radios. With over 200 million microwave ovens in use throughout the world, they are by far the greatest and most significant source of noise in this band. In addition microwave lighting/illumination systems are soon to be in use in the same band creating additional noise interference. Some examples of locations where it would be desirable to transmit images in the presence of periodic microwave noise are in the home (particularly in the kitchen), or in a supermarket or department store where a photo kiosk may be located near a microwave oven or lighting system.
One possible way to avoid the microwave is to use a spread spectrum communication technique. One type of spread spectrum technique spreads the signal over a band which is much larger than the bandwidth of the signal so that the narrow band noise from the noise source has a reduced effect on the demodulated signal to noise ratio. This technique however is relatively expensive to implement, significantly limits the data throughput (e.g. by a factor of 8 or more) and does not work well if the receiver is located very close to the noise source. Another way to avoid the noise described is demonstrated in U.S. Pat. No. 5,574,979, issued Nov. 12, 1996 to West, entitled xe2x80x9cPeriodic Interference Avoidance in a Wireless Radio Frequency Communication Systemxe2x80x9d. This patent demonstrates a potential solution by avoiding microwave oven periodic noise by sensing the periodicity of the AC power line main in which the oven is connected to. Unfortunately this technique does not work for periodic noise sources other than microwave ovens or in cases where multiple microwave noise sources on different phases exist. This technique also does not work in cases where the RF communication equipment is battery powered and no connection to an AC power main can be made, nor can the AC main E field radiation be received by the RF communication equipment.
There is a need therefore for an improved means of digital communication in environments with periodic noise and particularly those environments involving the 2.4 to 2.5 GHz ISM band.
The problem is solved according to the present invention by providing a digital communication system that can communicate in the presence of periodic noise by detecting the quiescent periods in the periodic RF radiated noise emissions and mapping and storing the envelope of this noise. The communication system then uses the internally stored mapped noise envelope to correctly time all communication such that digital data is sent only during the quiescent periods of the noise.
The advantages of the present invention are:
1. The RF link does not have to change frequency or use any correlation techniques in order to mitigate the periodic noise.
2. The software and clock synchronization circuitry required to execute the off duty cycle transfer has significant cost savings and smaller size requirements than the expensive and bulky circuitry currently used for the prior art spread spectrum techniques described above.
3. During the presence of 50% duty cycle periodic noise, image data transfer can be maintained at an average rate of approximately one half the normal through put rate.
4. The system can be battery powered and need have no connection in any way to an AC main in order to obtain the appropriate timing for periodicity.